2330 - Dosimetric Evaluation of Large-Area Proton Minibeam Radiation Therapy System for Clinical Applications

J. Kim¹, D. Kim², S. H. Choi II³, U. J. Hwang⁴, K. Park⁵, K. Jo⁶, M. Kim⁷, H. Kim⁸, H. S. Kim², C. Kim², J. H. Jeong², D. Shin⁹, S. B. Lee², T. H. Kim¹⁰, and Y. K. Lim¹¹; ¹Proton Therapy Center, National Cancer Center, Goyang, BUS, Korea, Republic Of, ²Proton Therapy Center, National Cancer Center, Goyang, Korea, Republic of (South), ³Research Team of Radiological Physics & Engineering, Korea Institute of Radiological & Medical Sciences, Seoul, Korea, Republic of (South), ⁴Department of Radiation Oncology, Chungnam National University Sejong Hospital, Daejon, Korea, Republic Of, ⁵Department of Radiation Oncology, Asan Medical Center, Seoul, Republic of Korea, Seoul, Korea, Republic of (South), ⁶Department of Radiation Oncology, Samsung Medical Center, Seoul, Korea, Republic of (South), ⁷Dongsan Hospital, Daegu, Korea, Republic Of, ⁸Department of Radiation Oncology, Yonsei Cancer Center, Heavy Ion Therapy Research Institute, Yonsei University College of Medicine, Seoul, Korea, Republic of (South), ⁹Proton Therapy Center, National Cancer Center, Goyang 10408, Korea, Republic of (South), ¹⁰Center for Proton Therapy, Research Institute and Hospital, National Cancer Center Korea, Goyang, Korea, Republic of (South), ¹¹Proton Therapy Center, National Cancer Center, Goyang 410-769, Korea, Republic of (South)

Purpose/Objective(s): To assess the dosimetric properties of a large-area proton minibeam radiation therapy (pMBRT) system for treating static solid tumors, including those in the central nervous system (CNS)

Materials/

Methods: The large-area (30x40 cm2) pMBRT system was designed to encompass the entire brain in CSI and to be compatible with our proton therapy system (IBA’s Proteus-235). The pMBRT system included a multi-slit collimator (MSC), a depth-dose modulator, a neutron absorber, a range shifter, and a rectangular snout. The MSC generates a set of narrow minibeams due to 1.5 mm-wide slits, and the depth-dose modulator converts proton minibeams into a broad beam in the tumor. The peak dose uniformity and the peak-to-valley dose ratio (PVDR) values of the proton minibeams were measured using radiochromic films when the front end of the system was positioned at 10 cm and 20 cm from the isocenter. Additionally, the depth-dose modulator was tested for its ability to adjust PVDR value.

Results: All components of the pMBRT system were successfully fabricated and installed into a proton pencil beam scanning nozzle at our proton therapy facility. The measured peak dose uniformity of the proton minibeams was approximately 3.1% and 3.9% when the front end of the system was positioned at 10 cm and 20 cm from the isocenter, respectively. Correspondingly, PVDR values exceeding 400 were measured at the phantom surface. At depths of 1 cm, 5 cm, and 10 cm in the phantom, the PVDR values changed to 124.5, 8.3, and 1.3, respectively. The depth-dose modulator actively adjusted the PVDR in depth to achieve uniform dose in the target region.

Conclusion: The pMBRT system, utilizing a large-area MSC, has been developed, and its dosimetric properties have been evaluated. Its field size was large enough to cover the entire brain in CSI. The uniform peak doses contribute to a uniform tumor dose, while a high PVDR, exceeding a few tens on patient skin, may dramatically reduce radiation toxicity, including hair loss, on the skin.

Abstract 2330 - Table 1: Dosimetric properties of large-area (30x40 cm²) MSC for pMBRT

1

Distance from pMBRT system to Isocenter	Depth in phantom	PVDR	Dose uniformity at peaks
20 cm	0 cm (surface)	= 400	3.1 %
	1 cm	124.5	-
	5 cm	8.3	-
	10 cm	1.3	-
10 cm	0 cm (surface)	= 400	3.9 %